1. Using a Multimeter, Test Light, and Other Test Equipment
Unit 3
Using a Multimeter, Test Light, and Other Test Equipment

2. Introduction
Electrical measuring tools to diagnose and repair electrical faults:
Digital volt-ohmmeters
Test lights
Fused jumper leads
Inductive amperage clamp

3. Digital Volt-Ohmmeter (1 of 5)
DVOM
Numerical reading on digital display
Likely to be first test tool selected

4. Digital Volt-Ohmmeter (2 of 5)
Basic DVOMs measure:
AC and DC voltage
AC and DC amperage
Resistance
Most also measure:
Frequency
Temperature
Have dedicated diode test capability

5. Digital Volt-Ohmmeter (3 of 5)
A variety of layouts and quality
Learn capabilities and how to use ones in shop.
Most average-quality ones are “fused.”
If amperage too high, fuse blows to protect meter.
If not fused, meter is not protected.

6. Digital Volt-Ohmmeter (4 of 5)
DVOMs and test leads have CAT ratings.
Each category designed to work safely on higher-powered electrical systems
Designed for high voltages
Hybrids—usually CAT III or CAT IV
Electric vehicles

7. Digital Volt-Ohmmeter (5 of 5)
When working on high-voltage systems:
Wear certified and tested rubber-insulated gloves.
Use proper CAT-rated meter and leads.
Use proper personal protective equipment.

8. Digital Volt-Ohmmeter—Use
Can take many different measurements
Measures electrical voltage within circuits (as voltmeter)
Measures resistance of:
Component
Connector
Cable

9. Digital Volt-Ohmmeter—Components (1 of 2)
Two components
Main instrument body
Test leads connecting to circuit
Main instrument body has:
Function switch
Digital display
Sockets

10. Digital Volt-Ohmmeter—Components (2 of 2)
Leads—pairs of one red, one black
Basic leads—probe on one end, connector on other
Wide variety of test leads and adapters
Alligator clips
Temperature probes
Inductive current clamps

11. Digital Volt-Ohmmeter— Ranges and Scales (1 of 3)
DVOMs read very small quantities in resistance measurements.
In range of millions of units
Impossible to accurately measure with single range of scale
Screen only displays four or five digits.

12. Digital Volt-Ohmmeter— Range and Scales (2 of 3)
Symbols are substituted for some digits.
Electrical symbol (V, A, or Ω) is placed behind factor symbol.

13. Digital Volt-Ohmmeter— Range and Scales (3 of 3)
Most DVOMs have both automatic and manual ranging capabilities.
Auto range—DVOM selects best range for value.
Meter does not give flashing warnings if range changes.

14. Digital Volt-Ohmmeter— Min/Max and Hold Setting (1 of 4)
Special settings assist in measuring rapidly changing values or freeze displays.
Setting records in memory the minimum and maximum reading while connected to source.

15. Digital Volt-Ohmmeter— Min/Max and Hold Setting (2 of 4)
Used to measure vehicle battery voltage while engine cranks or battery charges
Current highest during cranking, but only fraction of a second
Battery lowest when cranking
Min/max mode captures those numbers.

16. Digital Volt-Ohmmeter— Min/Max and Hold Setting (3 of 4)
Sample rate—the speed at which DVOM can sample voltage
DVOM checks voltage at regular intervals.
Transient voltage occurring between samples may not be recorded.
Use other tools if quicker sample rates needed.

17. Digital Volt-Ohmmeter— Min/Max and Hold Setting (4 of 4)
Hold function allows display to freeze.
Display holds value until function or DVOM turned off.
“Auto hold” capabilities on some DVOMs
Useful when difficult to watch display while making connections

18. Digital Volt-Ohmmeter— Setting Up a DVOM (1 of 2)
Need to know:
If measuring resistance, voltage, or current
The expected reading
Will help determine:
Connections to make
Range to select

19. Digital Volt-Ohmmeter— Setting Up a DVOM (2 of 2)
To set up a DVOM:
Select leads and probes needed for task.
Connect leads to DVOM.
Use function switch to select measurement type.
Select correct meter range (for manual).
Connect leads to circuits being tested.
Read meter display.

20. Digital Volt-Ohmmeter—Test Leads: Common and Probing (1 of 3)
Meter near test lead terminals labeled with:
A (typically 10 A) mA
Common
Common to all functions of meter
V/Ω
Red lead does move, depending on function

21. Digital Volt-Ohmmeter—Test Leads: Common and Probing (2 of 3)
When various electrical signals are measured at same time on an oscilloscope, need more than just red lead.
Yellow, blue, and green test leads also act as probes.
“Probing lead” is more accurate than “positive lead.”

22. Digital Volt-Ohmmeter—Test Leads: Common and Probing (3 of 3)
Meter screen reads what probing lead touches.
If probing lead touches positive post, meter screen will display “+” before reading.
If it touches a negative post, will display “–.”

23. Digital Volt-Ohmmeter— Probing Techniques (1 of 3)
Different types of probes:
Alligator clips
Fine-pin probes
Insulation piercing clips
(High voltage needs special probes)

24. Digital Volt-Ohmmeter— Probing Techniques (2 of 3)
Do not use excessive force when probing.
Standard probe leads:
Basic straight metal probes
Require both hands to hold in place

25. Digital Volt-Ohmmeter— Probing Techniques (3 of 3)
Leads with alligator clips
Can clip on circuit, freeing up hands
Useful for connecting to larger terminals
Back-probing—pushed through insulation from back of connector
Fine-pin probes used to reduce damage possibility
Always reinsulate.

26. Digital Volt-Ohmmeter—Measuring Volts, Ohms, and Amps (1 of 7)
Most common measurements:
Voltage
Resistance
Current

27. Digital Volt-Ohmmeter—Measuring Volts, Ohms, and Amps (2 of 7)
For voltage measurements:
Connect probing lead to V/Ω terminal.
Connect common lead to COM terminal.
Select range or auto range.
Probing lead is usually connected to positive side.
Common lead is usually connected to negative side.

28. Digital Volt-Ohmmeter—Measuring Volts, Ohms, and Amps (3 of 7)
Most measure milliamps or 10 to 20 amps.
Select correct range.
Connect red probe to A terminal.
Connect black probe to COM terminal.
May have a separate mA terminal to measure milliamps

29. Digital Volt-Ohmmeter—Measuring Volts, Ohms, and Amps (4 of 7)
Measuring current
Connect DVOM in series with the circuit.
Probing lead closest to positive terminal
DVOM may have internal fuse that blows if current is too high.

30. Digital Volt-Ohmmeter—Measuring Volts, Ohms, and Amps (5 of 7)
Measuring larger amperage
Connect clamps to measure high currents.
Fasten current clamp around conductor to measure magnetic field strength in current flowing through conductor.
Clamps instead of breaking into circuit to insert DVOM

31. Digital Volt-Ohmmeter—Measuring Volts, Ohms, and Amps (6 of 7)
Measuring component resistance
Remove component from circuit.
Disconnect power.
If not, may get false reading or damage the DVOM.

32. Digital Volt-Ohmmeter—Measuring Volts, Ohms, and Amps (7 of 7)
Measuring resistance
Connect red lead to the V/Ω terminal.
Connect black lead to the COM terminal.
Select range or auto range.
Red lead connected to one side
Black lead connected to other side

33. Voltage Exercises
Designed to explain use of DVOM in taking DC voltage measurements
Use of different ranges on the meter display
Voltage drops in series circuits across equal and unequal loads
Kirchoff’s voltage law—sum of series voltage drops equals the supply voltage

34. Voltage Exercises—Ranges (1 of 2)
DVOMs have capability for:
Auto range
Manual range
DVOMs have different range settings.
6 V, 60 V, and 600 V
4 V, 40 V, and 400 V

35. Voltage Exercises—Ranges (2 of 2)
A circuit with two resistors in a series with a 12-volt DC supply
Compare ranges by measuring voltage drops across each resistor.

36. Voltage Exercises— Voltage Drop (1 of 12)
Measured with a voltmeter
Potential difference between two points in a circuit
Sum of all voltage drops in a series circuit equals the supply voltage.

37. Voltage Exercises— Voltage Drop (2 of 12)
Voltage drop across all parallel circuit branches is the same.
Does not occur in all parts of the circuit
Vast majority is across the component or load that does the work.

38. Voltage Exercises— Voltage Drop (3 of 12)
Unwanted voltage drop may be excessive.
May be in other parts of circuit besides the load
Example: Only resistance should be in headlight bulb
Resistance also in cable and connectors
If resistance high here, circuit efficiency is low

39. Voltage Exercises— Voltage Drop (4 of 12)
Excessive voltage drop—fault in circuit
To test for unwanted voltage drop:
Measure voltage drop across each circuit part and add voltage drops together.
In 12-volt system—total drop across each side should not exceed 0.5 (in 24-volt, 1.0 volt)

40. Voltage Exercises— Voltage Drop (5 of 12)
To measure voltage drop:
Set DVOM on the voltage range.
Set function switch to “auto range volts DC.”
Connect black lead to COM.
Connect red lead to V/Ω.

41. Voltage Exercises— Voltage Drop (6 of 12)
Can be measured across:
Components
Connectors
Cables
Probing lead normally connected to circuit point where voltage needs to be checked

42. Voltage Exercises— Voltage Drop (7 of 12)
Example: Voltage drop test on feed side of horn circuit
Connect black lead to positive terminal.
Connect red lead to input wire of horn.
When horn is activated, voltmeter reads amount of voltage drop in feed side.

43. Voltage Exercises— Voltage Drop (8 of 12)
If –4.2 volts, voltage is 4.3 volts less at input of horn than at positive battery post.
Drop more than 0.5 volts—excessive voltage drop
Check wire by wire until voltage drop point is located.

44. Voltage Exercises— Voltage Drop (9 of 12)
Same measurements can be done with DVOM leads reversed.
Red lead on positive battery post
Black lead on input of horn
Meter will read 4.2 volts, showing positive.

45. Voltage Exercises— Voltage Drop (10 of 12)
Voltage in various parts of the circuit are measured with the switch in open position.

46. Voltage Exercises— Voltage Drop (11 of 12)
Series circuit of two resistors with a 12-volt battery and switch
Voltage in various areas is measured with switch in closed position.

47. Voltage Exercises— Voltage Drop (12 of 12)
Unwanted voltage drops in circuits
Corroded or bad chassis ground reduces voltage and current available.
Simple circuit with a bulb connected via a switch across a 12-volt circuit.

48. Voltage Exercises—Voltage Drop Across Multiple Loads (1 of 2)
DVOM on voltage range
Select “auto range volts DC.”
Connect black lead to COM.
Connect red lead to V/Ω.
Current needs to flow for accurate measurement.
Leads can be placed in either direction.

49. Voltage Exercises—Voltage Drop Across Unequal Loads (2 of 2)
Set DVOM on voltage range.
Select “auto range volts DC.”
Connect black lead to COM.
Connect red lead to V/Ω.
Measure voltage drop only when current flows.
Leads can be placed in either direction.

50. Current Exercise (1 of 4)
Exercise explains DVOM use for taking DC current measurements.
Discusses Ohm’s law and current measurements
Demonstrates measuring current
Shows magnetic fields around a conductor during current flow

51. Current Exercise (2 of 4)
Current is the same in all parts of a series circuit.
Ammeter must be connected in series.
To ensure all current flows through the ammeter:
Circuit must be broken in two.
Ammeter is connected to one of two broken ends.

52. Current Exercise (3 of 4) Voltage measurement: Current measurement:
Select “auto range amps DC.”
Connect red lead to A socket.
Connect black lead to COM.
Current measurement:
Select “auto range amps DC.”
Connect red lead to A socket.
Connect black lead to COM.

53. Current Exercise (4 of 4)
A circuit with a single resistor with a 12-volt DC supply
DVOM used to measure both voltage and current

54. Current Exercises— Measuring Current (1 of 2)
To measure current:
Select “auto range amps DC.”
Connect red lead to A socket.
Connect black lead to COM socket.
Select appropriate range if using manual.

55. Current Exercises— Measuring Current (2 of 2)
A circuit with two resistors in a series with a 12-volt DC supply
Connect DVOM to various circuit parts to measure current flow.

56. Current Exercises— Current and Magnetic Fields (1 of 3)
Example: Relay controlled by switch used to switch current through a resistor
Compass demonstrates a magnetic field is produced around relay winding when current flows through it.

57. Current Exercises— Current and Magnetic Fields (2 of 3)
To conduct experiment:
Set DVOM to measure “DC amps.”
Connect red lead to A socket.
Connect black lead to COM socket.
Select appropriate range if using manual.

58. Current Exercises— Current and Magnetic Fields (3 of 3)
Circuit with relay controlled by switch and single resistor with a 12-volt DC supply
Compass shows that when energized, relay winding produces magnetic field.
DVOM measures current flow.

59. Resistance Exercises (1 of 2)
Exercises show how to use DVOM to measure resistance.
Check components/circuits against specs.
Examples demonstrate:
Measuring resistance
How resistance affects current flow

60. Resistance Exercises (2 of 2)
Current flow is inversely proportional to resistance.
The higher the resistance, the less current that will flow.
The lower the resistance, the higher the current flow.

61. Measuring Resistance (1 of 3)
For resistance measurements:
Select “auto range Ω.”
Connect red lead to V/Ω.
Connect black lead to COM.
If manual, select by starting at highest range and working down.

62. Measuring Resistance (2 of 3)
A circuit with a lamp in series with a resistor and a 12-volt DC supply

63. Measuring Resistance (3 of 3)
DVOM is used to measure resistance.
The measurement to be expected from the circuit

64. Resistance Exercise 1 (1 of 3)
Measure resistance, voltage, and current. For resistance and voltage measurements:
Select “auto range volts DC.”
Connect red lead to V/Ω.
Connect black lead to COM.
Do measurements with component disconnected.

65. Resistance Exercise 1 (2 of 3)
For current measurements:
Select “auto range amps DC.”
Connect red lead to the A socket.
Connect black lead to COM socket.

66. Resistance Exercise 1 (3 of 3)
A circuit with a resistor and a 12-volt DC supply
DVOM measures resistance, voltage, and current.

67. Resistance Exercise 2 (1 of 3)
Measure resistance, voltage, and current.
For resistance and voltage measurements:
Select “auto range volts DC.”
Connect red lead to V/Ω.
Connect black lead to COM.
Disconnect component before measuring.

68. Resistance Exercise 2 (2 of 3)
For current measurements:
Select “auto range amps DC.”
Connect red lead to the A socket.
Connect black lead to COM socket.

69. Resistance Exercise 2 (3 of 3)
Circuit with resistor and a 12-volt DC supply

70. Resistance Exercise 3 (1 of 3)
Measure resistance, voltage, and current. For resistance and voltage measurements:
Select “auto range volts DC.”
Connect red lead to V/Ω.
Connect black lead to COM.
Disconnect component before measuring.

71. Resistance Exercise 3 (2 of 3)
For current measurements:
Select “auto range amps DC.”
Connect red lead to the A socket.
Connect black lead to COM socket.

72. Resistance Exercise 3 (3 of 3)
A circuit with a resistor and a 12-volt DC supply in a series with a LED
DVOM will measure resistance, voltage, and current through R1, a 100-ohm resistor.

73. Resistance Exercise 4 (1 of 3)
Measure resistance, voltage, and current For resistance and voltage measurements:
Select “auto range volts DC.”
Connect red lead to V/Ω.
Connect black lead to COM.
Disconnect component before measuring.

74. Resistance Exercise 4 (2 of 3)
For current measurements:
Select “auto range amps DC.”
Connect red lead to the A socket.
Connect black lead to COM socket.

75. Resistance Exercise 4 (3 of 3)
A circuit with a resistor and a 12-volt DC supply in series with an LED
Measures through R1, a 10-kΩ resistor

76. Electrical Circuit Testing
To diagnose electrical faults:
Understand circuit types and electricity.
Ability to use meters/oscilloscopes to measure:
Voltage
Amperage
Resistance
Ability to read wiring diagrams

77. Electrical Circuit Testing— Using Ohm’s Law (1 of 11)
Ohm’s law can be used to:
Perform math to predict and verify measurements
Use relationships demonstrated for diagnosis

78. Electrical Circuit Testing— Using Ohm’s Law (2 of 11)
Ohm’s law to calculate electrical quantities
Cross-checking measured results within circuit
Example: If resistance and voltage are known, can calculate theoretical current.
Calculated result can be compared to results measured with an ammeter.

79. Electrical Circuit Testing— Using Ohm’s Law (3 of 11)
Quick calculation is often done for an approximate value before actual measurements are taken.
Helps set the measuring tool to correct range
Calculations yield approximate values only.

80. Electrical Circuit Testing— Using Ohm’s Law (4 of 11)
Used to find relationships between:
Volts
Amps
Ohms
Example: If voltage stays the same but resistance decreases, amperage increases.

81. Electrical Circuit Testing— Using Ohm’s Law (5 of 11)
Ohm’s law regarding voltage changes
If voltage decreases and resistance stays the same, amperage will decrease.
If voltage increases and resistances stays the same, amperage increases.
If amperage and voltage both increase, electrical power increases.

82. Electrical Circuit Testing— Using Ohm’s Law (6 of 11)
Amperage is a result of voltage and resistance:
Cannot exist without them
Amperage does what voltage and resistance allow.
If amperage is low:
Voltage low
Resistance high
If amperage is high:
Voltage high
Resistance low

83. Electrical Circuit Testing— Using Ohm’s Law (7 of 11)
Amperage is a product of voltage and resistance.
If circuit fault, amperage will be either high or low.
In most cases, flow is obvious.
Fuse blown from too-high current
Light dimmed from too-low current

84. Electrical Circuit Testing— Using Ohm’s Law (8 of 11)
If current is low:
Either voltage is low or resistance is high.
Ohm’s law
If current is high:
Either voltage is high or resistance is low.

85. Electrical Circuit Testing— Using Ohm’s Law (9 of 11)
Example: Left front headlight is dim.
Current low, with either low voltage or high resistance
Voltmeter to measure voltage at battery
If low, determine why.

86. Electrical Circuit Testing— Using Ohm’s Law (10 of 11)
If voltage at battery is good:
Check voltage across both sides of headlight (with circuit on).
Should be within 1.0 volts of battery voltage
If not, look for high resistance.
Measure voltage drop on each side.
If excessive on one side, follow back toward battery to identify cause.
If voltage on both sides are within specs, problem likely headlight.
Compare to a known good bulb.

87. Electrical Circuit Testing— Using Ohm’s Law (11 of 11)
If current flow is too high:
Either too much voltage or too little resistance
Measure battery voltage.
Ohmmeter to check load resistance and compare to specs to find any shorts
If not, ohmmeter to check wire harness for short circuits

88. Electrical Circuit Testing—Using Wiring Diagrams to Diagnose Electrical Circuits (1 of 6)
Wiring diagrams or schematics available as:
Paper-based manuals
Computer programs
Online resources
Often repair information via Internet by regularly updated subscription services

89. Electrical Circuit Testing—Using Wiring Diagrams to Diagnose Electrical Circuits (2 of 6)
Must understand symbols, abbreviations, and connector coding
Like reading a road map:
Interconnect circuits
Wires
Components

90. Electrical Circuit Testing—Using Wiring Diagrams to Diagnose Electrical Circuits (3 of 6)
Circuits usually consist of:
Power source
Switch
Load
Ground

91. Electrical Circuit Testing—Using Wiring Diagrams to Diagnose Electrical Circuits (4 of 6)
Jorge Menchu color-crayon approach to understanding circuits in wiring diagrams

92. Electrical Circuit Testing—Using Wiring Diagrams to Diagnose Electrical Circuits (5 of 6)
On copy of wiring diagram, color:
Wires directly connected to ground—green
Wires that are “hot” all the time—red
Wires “switched to power”—orange
Wires “switched to ground”—yellow
Wires that reverse polarity—side-by-side orange and yellow lines
Variable wires—blue

93. Electrical Circuit Testing—Using Wiring Diagrams to Diagnose Electrical Circuits (6 of 6)
Coloring wires on wire diagram:
Forces determination of what each wire does
Helps organize and understand how electricity flows through circuit
Helps prevent losing place or forgetting what a wire does
Helps in diagnosis

94. Electrical Circuit Testing—Using a DVOM to Measure Voltage (1 of 12)
In most measurements, set to auto range
Select leads and probe ends to match task.
Example: If needing hands free, use alligator clips.
Do not exceed maximum allowable voltage or current for DVOM.

95. Electrical Circuit Testing—Using a DVOM to Measure Voltage (2 of 12)
Use appropriate personal protective equipment for high voltages.
High-voltage safety gloves
Long-sleeved shirt and pants
Protective eyewear
Remove items that may cause short circuits.

96. Electrical Circuit Testing—Using a DVOM to Measure Voltage (3 of 12)
Simple voltage test
Place common lead on good ground.
Place probing lead on input side.
Gives reading on amount of voltage at probing lead
Does not determine voltage started with or how much did not make it through the circuit

97. Electrical Circuit Testing—Using a DVOM to Measure Voltage (4 of 12)
Voltage drop test
Voltage drop occurs when current flows through a resistance.
Higher the resistance, higher the voltage drop
Measures for excessive resistance
Always have current turned on.

98. Electrical Circuit Testing—Using a DVOM to Measure Voltage (5 of 12)
Two ways to do a voltage drop test
Direct method—both test leads on same side of circuit
Directly reads how much voltage is lost between two points

99. Electrical Circuit Testing—Using a DVOM to Measure Voltage (6 of 12)
Two voltage drop tests (cont’d)
Indirect method— black lead on negative battery terminal at all times
Probing lead moved from one point to another
Second reading subtracted from first reading

100. Electrical Circuit Testing—Using a DVOM to Measure Voltage (7 of 12)
Placing leads in circuit for direct voltage drop test
If checking positive side of circuit:
Probe with red lead.
Place black lead on positive battery post.

101. Electrical Circuit Testing—Using a DVOM to Measure Voltage (8 of 12)
Example: Measure voltage drop on positive side of low beam filament on left headlight.
Voltmeter reads –0.71 on low beam.
Voltage is 0.71 less than voltage at black lead.
0.5-volt drop is maximum allowed.

102. Electrical Circuit Testing—Using a DVOM to Measure Voltage (9 of 12)
Example: Check voltage drop on ground side of circuit.
Place black lead on output terminal of headlight.
Place red lead on negative battery terminal.
Turn on low beam light, measure voltage drop.
Meter reads –0.24 volts.

103. Electrical Circuit Testing—Using a DVOM to Measure Voltage (10 of 12)
To check a different way:
Place black lead on negative battery terminal.
Place red lead on output side of headlight.
Turn headlight on and take reading.
Measures 0.25 volts

104. Electrical Circuit Testing—Using a DVOM to Measure Voltage (11 of 12)
Indirect method:
Take two voltage readings.
Subtract them from each other to determine drop.
Useful when:
Working far from battery
Connecting voltmeter leads on battery not possible

105. Electrical Circuit Testing—Using a DVOM to Measure Voltage (12 of 12)
Base reading: Measure voltage at battery with electrical device on.
Second measurement is taken at load being tested, circuit on.
Subtract voltage at load from battery voltage
Difference is voltage drop.

106. Electrical Circuit Testing—Checking Circuits with a Test Light (1 of 3)
Nonpowered test lamps determine if electrical power is present in part of circuit.
Test the test light on good power and a ground; if not:
Circuit missing one or both elements
Test light faulty

107. Electrical Circuit Testing—Checking Circuits with a Test Light (2 of 3)
Good for simple tests
Test light lead can be grounded quickly and probe end touched to each end of fuse.
If both light, fuse is good.
If only one end lights, fuse is blown.

108. Electrical Circuit Testing—Checking Circuits with a Test Light (3 of 3)
Circuit voltage is not to exceed test light rating.
Most test lights are rated for 6- to 12-volt system.
Do not use a test light to test SRS.
Using test light on computer circuit designed for very small current flows can damage the circuit.

109. Electrical Circuit Testing—Checking Circuits with Fused Jumper Leads (1 of 2)
Can be used to assist in checking circuits
Can be created or purchased in range of:
Sizes
Lengths
Fittings (connectors)

110. Electrical Circuit Testing—Checking Circuits with Fused Jumper Leads (2 of 2)
Used to extend connections to allow circuit readings or tests with:
DVOM
Oscilloscope
Current clamps on fuses
Relays or connector plugs

111. Electrical Circuit Testing—Locating Opens, Shorts, Grounds, and High Resistance (1 of 2)
Tools most often used to locate opens, shorts, grounds, and high resistance:
DVOMs
Test lamps
Simulated loads

112. Electrical Circuit Testing—Locating Opens, Shorts, Grounds, and High Resistance (2 of 2)
Open circuit—break in electrical circuit
Power supply or ground circuit interrupted
Often can be located by:
Probing along various points of circuit
Checking for effective grounding at ground point

113. Electrical Circuit Testing— Locating Open Circuits (1 of 2)
Systemic circuit check required
Voltage drop check on each side of circuit
Will cause voltage drop equal to source voltage
Once side of drop is found:
Continue voltage drop testing on that side by working leads closer together in steps.

114. Electrical Circuit Testing— Locating Open Circuits (2 of 2)
Consider most likely places for open circuit.
Blown fuse
Faulty switch
Open load
If within specs, check for open load with ohmmeter or manufacturer diagnostics.

115. Electrical Circuit Testing— Locating Short Circuits (1 of 3)
Short circuit—circuit fault in which current travels along an accidental/unintended route
Can occur anywhere in the circuit
May be intermittent
May occur within load or in wiring

116. Electrical Circuit Testing— Locating Short Circuits (2 of 3)
Causes lower-than-normal resistance
Abnormally high current flow
Circuit protection devices open circuit.
Circuit may remain live after switched off.
Possible causes: Faulty components or damaged wiring

117. Electrical Circuit Testing— Locating Short Circuits (3 of 3)
Best tested by comparing ohmmeter reading to specifications
Shorts in wire harnesses best tested by:
Disconnecting each end
Using ohmmeter to test for unwanted continuity between wires

118. Electrical Circuit Testing— Locating Grounds (1 of 2)
Grounds—reference to short to ground
Conduct initial test by resistance checks or disconnecting the load.
Example: To test blower motor, disconnect it.
If short still in place, wiring between fuse or circuit breaker and load is at fault.

119. Electrical Circuit Testing— Locating Grounds (2 of 2)
Connect a test lamp or buzzer in place of a fuse and find a ground through the short.
Disconnect parts of circuit to narrow location.
Specialized short circuit detection tools
Send signal through wiring harness
Receiving device moves along wire loom to indicate location.

120. Electrical Circuit Testing— Short to Power
Short to power—when power from one circuit leaks into another circuit
Causes strange electrical issues:
One or more circuits operate when they shouldn’t
In sensor wires, incorrect signals cause computer to receive faulty data.

121. Electrical Circuit Testing— High Resistance (1 of 3)
High resistance—circuit with unintended resistance
Can be caused by:
Corroded or loose harness connectors
Incorrectly sized cable for circuit current flow
Incorrectly fitted terminals or poorly soldered joints

122. Electrical Circuit Testing— High Resistance (2 of 3)
Causes unintended voltage drop when current flows
Reduces amount of voltage available to the load
Reduces current flow in the circuit
Reduction in voltage and current to the load reduces the amount of electrical power to load.

123. Electrical Circuit Testing— High Resistance (3 of 3)
Locate by voltage drop testing in power and ground circuits.
If within the load:
Check resistance with ohmmeter.
Some devices may need further testing.

124. Electrical Circuit Testing—Inspecting and Testing Circuit Protection Devices (1 of 4)
Designed to prevent excessive current from flowing in the circuit
Sacrificial—will blow or trip to prevent excessive current flows
Fuses
Fusible links

125. Electrical Circuit Testing—Inspecting and Testing Circuit Protection Devices (2 of 4)
Circuit breakers can be reset—either automatically or manually.
Available in various ratings, types, and sizes
Must be replaced with the same rating and type

126. Electrical Circuit Testing—Inspecting and Testing Circuit Protection Devices (3 of 4)
Usually situated in power or feed side
Fuses are tested with DVOM or test lamp.
Good fuse—virtually same voltage on both sides
Blown fuse—battery voltage on one side and 0 volts on the other

127. Electrical Circuit Testing—Inspecting and Testing Circuit Protection Devices (4 of 4)
Can sometimes be visually inspected
Fusible element should be intact.
Should be very low resistance when measured with ohmmeter
Contacts on fuse and holder should be clean and free of corrosion.

128. Inspecting and Testing Switches, Connectors, Relays, Solenoid Solid-State Devices, and Wires (1 of 5)
Start with visual inspection of electrical circuit, then electrical testing.
Visually inspect for:
Breakage or corrosion
Deformity
Worn or melted insulation

129. Inspecting and Testing Switches, Connectors, Relays, Solenoid Solid-State Devices, and Wires (2 of 5)
Electrical inspection necessary for:
Switches
Solenoid contacts
Relay contacts
Example: Switches require voltage drop testing to check for excessive resistance.

130. Inspecting and Testing Switches, Connectors, Relays, Solenoid Solid-State Devices, and Wires (3 of 5)
Some solenoids can be disassembled for visual inspection.
If excessive voltage drop across contacts, will be pitted and burned
Measure resistance if shorted relay or solenoid winding is possible.

131. Inspecting and Testing Switches, Connectors, Relays, Solenoid Solid-State Devices, and Wires (4 of 5)
DVOMs and test lamps for basic testing
More specialized test equipment if needed
Test lamps can overpower electronic components—should not be used for them.
Resistance tests can be conducted on components in or out of the circuit.

132. Inspecting and Testing Switches, Connectors, Relays, Solenoid Solid-State Devices, and Wires (5 of 5)
Diagnostic flowcharts from manufacturers show sequence based on test results.
Complex circuits—gather information on circuit operation and customer concern to formulate a testing sequence.

133. Summary
The digital volt-ohmmeter (DVOM) or digital multimeter (DMM) is an electrical measurement tool used to diagnose and repair electrical faults.
To properly use a DVOM requires time and effort to learn the parts and how it works.
The DVOM can measure volts, ohms, and amps in a circuit.

134. Summary
An advanced DVOM measures frequency and temperature, and has a dedicated diode test capability.
A DVOM is the first tool used to take electrical measurements.

135. Summary
The DVOM allows the technician to see the movement of electrical impulses that cannot be seen without some type of electrical test equipment.
The DVOM can measure electrical volts within circuits.
The DVOM can measure ohms, which is the resistance of a circuit.
The DVOM can measure amps, which is the current flow of a circuit.

136. Summary
The main parts of the DVOM are the main body and the two current leads.
The main body has a function switch, a connection point for the leads, and a digital display to show values.

137. Summary
The leads are red for positive and black for negative connections.
There is a wide selection of leads for the DVOM to enhance the testing capabilities.
Before using a DVOM, the technician needs to know the quantity of the measurement (volt, ohm, or amp).

138. Summary
The DVOM can read a wide range of scales depending on the position selected.
The DVOM can read from low to high values.
The DVOM in auto range will select the best value for the range being measured.

139. Summary
The min/max setting gives the technician the ability to measure circuits that are only on momentarily.
The hold function freezes the value measured.

140. Summary
There are many different ways to probe a circuit depending on the circuit being tested.
The probes should never be forced as this could damage the circuit being tested and the probes being used.
If the technician uses the back probe method, the holes probed need to be resealed to keep moisture out.

141. Summary
The most common measurements taken with a DVOM are voltage, current, resistance.
Depending on measurements taken, the leads need to be in the correct location on the body of the DVOM.
If the leads are connected in the wrong place on the DVOM, it could cause a fuse to blow.

142. Summary
When voltage is measured, the leads are placed parallel to the circuit being measured.
When current is measured, the leads are placed in series with the circuit being measured.
When resistance is measured, the component should be isolated from the circuit so no power is present.
The meter is very useful in finding opens, shorts, grounds, and high resistance.

143. Credits
Unless otherwise indicated, all photographs and illustrations are under copyright of Jones & Bartlett Learning.